Monolithic catalytic converters typically include an extruded ceramic honeycomb structure of a refractory material such as cordierite (2Mg0.5SiO.sub.2.2Al.sub.2 O.sub.3) or mullite (3Al.sub.2 O.sub.3.2SiO.sub.2). This ceramic material is wash coated with a thin layer of a catalyst carrier such as alumina (Al.sub.2 O.sub.3) or zirconium oxide (ZrO.sub.2) of very high surface area. The high surface area carrier is usually impregnated with a noble metal such as platinum (Pt), palladium (Pd), rhodium (Rh) or a mixture thereof. After the honeycomb structure and the catalyst are heated up to the activation temperature, noxious components of the automobile exhaust gas such as unburned hydrocarbons (UHC), carbon monoxide (CO) and nitrogen oxides (NOx) react in the catalyzed reaction forming harmless gaseous products.
Each catalyst material has a light-off temperature at which the rate of the catalyzed reaction increases from very low to very high levels. In designing a monolithic converter for automobile emission controls, it is desirable to have a honeycomb structure in such a geometry that it can be heated up quickly during the cold start of a vehicle.
A potential problem in the use of monolithic honeycomb structure converters is the melt-down of the monolithic core at very high temperatures. This occurs due to the combined effect of the following two causes. First, high temperatures are generated in each channel at or near the catalyzed reaction sites on the channel walls by the extremely exothermic chemical reactions in the exhaust gas. Secondly, in present monolithic converters having uniformly cross sectioned flow channels, the exhaust gas flow velocity in each channel is the same. This means that the maximum temperature, i.e., a hot spot, occurs at the same longitudinal location in each channel. The cumulative effect of all the hot spots in the same region of the monolith can produce a melt-down of the monolithic core. The probability of occurrence of a melt down is higher when an automobile is operated under highly unstable conditions. For instance, during the deceleration phase of a vehicle or the malfunction of a spark plug, a sudden increase in the reactant concentration (UHC, CO, NOx) greatly increases the maximum temperature at localized hot spots.
The problems of uneven heating and subsequent thermal shock occurrence have been recognized by other workers in monolithic converters. Various means were proposed to remedy the effect of thermal shock. One of such means was to design uniform non-square shaped honeycomb cells so that the cell walls could withstand higher thermal stresses and provide improved thermal shock resistance. Another was to provide discontinuities in the cell walls extending longitudinally through the structure and transversely through the cell walls so that the monolith could better withstand uneven thermal expansion. However, much of these efforts were remedial in nature to lessen the effect of the thermal shock rather than preventing it from occurring. Moreover, the concept of the hot spot formation was generally not recognized by others in the field.